1. Field of the Invention
The present invention relates to systems and methods for controlling acoustical damping of intake air sound or exhaust air sound of vehicle engines.
2. Description of the Related Art
A Helmholz resonator is known as an acoustical damping device. As shown in FIG. 4, a known Helmholz resonator 90 is configured by a branch tube 94 connected to an intake air duct 92 of an engine and by a closed chamber 96 connected to the branch tube 94. Damping frequency or resonance frequency f of the resonator 90 is determined by cross-sectional area S of the branch tube 94, length L of the branch tube 94 and volume V of the chamber 96, and is represented by:
  f  =            c              2        ⁢        π              ⁢                  S        LV            
where c is the sonic velocity (i.e., 340 m/s). In the case of intake air noise that may be produced due to the pulsation of the intake air supplied to an engine, the noise level may have a peak value at a specific frequency F corresponding to the rotational speed of the engine. Therefore, in principle, the noise level at the specific frequency F may be reduced by controlling the resonance frequency f of the resonator 90 to coincide with the specific frequency F of the intake air noise corresponding to the rotational speed of the engine. Thus, the intake air sound may be generally reduced overall because the intake air sound at the frequency F, at which the sound level has a peak value, may be reduced. The resonator 90 may also reduce the exhausted air noise in the same way.
In order to have the resonance frequency f coincide with the frequency F of the intake air sound (or exhaust air sound) corresponding to the rotational speed of the engine, Japanese Laid-Open Patent Publication No. 5-288033 teaches the ability to change the length L of the branch tube 94 and the cross-sectional area S of the branch tube 94.
As noted above, the resonator 90 is generally used for minimizing the intake air sound or the exhaust air sound of the engine. However, during traveling for the sake of enjoying sporty driving, it is not possible to obtain a vigorous feeling or enthusiastic feedback if the intake air sound or the exhaust air sound is low. The intake or exhaust air sound may not be loud enough for a driver or a passenger to enjoy during these situations. Consequently for such an occasion, it is preferable to increase the intake air sound or the exhaust air sound by an appropriate degree. Conversely, it is also preferable to minimize the intake air sound or the exhaust air sound during the playing of audio equipment or when attempting to have a quiet conversation.